Classical methods for determining the presence and number of bacteria in a sample are time consuming, tedious and labor intensive. Typically, a technician must prepare reagents and nutrients, mix the nutrients with agar, heat the mixture, pour the mixture into a petri dish, allow the agar to gel, obtain a test sample, dilute the test sample, add an aliquot of the diluted sample to the agar, incubate the inoculated plate for 24-48 hours and finally count the number of growing bacterial colonies in the petri dish. Products and processes which reduce the preparation time and which facilitate a count of the bacterial colonies would clearly be welcomed by those working in this field.
One example of a product which greatly simplifies the above preparation time is a thin film culture plate device for growing microorganisms that is described in U.S. Pat. No. 4,565,783 as well as variations of this device such as those described in U.S. Pat. Nos. 5,089,413; 5,137,812 and 5,232,838. In a typical thin film culture plate device, a reconstitutable dry powder containing a gelling agent and microbial growth nutrients is coated on a waterproof substrate. A transparent, read-through cover sheet coated on a surface with an acrylate adhesive containing an indicator dye and powdered gelling agent is attached to the coated substrate.
When the above device is used, a predetermined amount of an aqueous sample is typically placed in contact with the coated substrate and the cover sheet is placed over the sample and substrate. The aqueous sample hydrates the soluble dry powder which then forms a gelled medium capable of sustaining microbial growth. During the growth period, the indicator dye adhered to the cover sheet reacts in the presence of viable microorganisms to give a detectable response that allows visualization of bacterial colonies which are grown on the culture device. Thin film culture plate devices are commercially available and are sold under the tradename PETRIFILM plates by 3M, St. Paul, Minn.
Thin film culture plate devices are generally much simpler to use than conventional agar medium/petri dish systems because there is no need for the user to heat and mix the growth medium, agar and other reagents and then add the mixture to petri dishes or pour plates. In addition, these devices are compact and easily disposed of and therefore are easier and safer to use.
In spite of the many advantages that thin film culture plate devices have over conventional types of culture systems, the utility of these thin film plates for certain applications may be challenged under certain conditions, microbes and samples. For example, the limited amount of gelling agent in the device may be insufficient to remain in a semi-solid state when inoculated with certain samples containing some microorganisms. Briefly, some samples which contain certain bacteria commonly known as "liquifiers" will cause the growing colonies to overrun the semi-solid gel and thus hinder detection and enumeration of such microorganisms.
The present invention addresses the difficulties presented in attempting to grow, detect and enumerate a wide variety of microorganisms using thin film culture plate devices.